Current demands for a low fuel consumption and high engine performance from internal combustion engines intended for vehicles have led to ever more powerful supercharging systems. Supercharging systems working in parallel and in tandem, such as two exhaust gas turbines arranged in parallel, where one operates at low engine speeds and the other exhaust gas turbine is only connected at high engine speeds, are already known. One advantage of these systems is that they afford good low-speed characteristics when small volumes of exhaust gas are directed exclusively to the first exhaust gas turbine and good high-speed characteristics when large volumes of exhaust gas are directed to both of the exhaust gas turbines.
The United States patent documents U.S. Pat. Nos. 4,993,228 and 5,201,790 show this type of supercharging system, which comprises a turbo unit having a first and a second exhaust gas turbine coupled to a first and a second exhaust manifold, respectively. These exhaust manifolds are arranged independently of one another in respect of each cylinder, that is the first exhaust manifold is connected to a first exhaust port in each cylinder, while the second exhaust manifold is connected to a second exhaust port in each cylinder. At partial load only the first exhaust gas turbine is connected, while the second exhaust gas turbine is connected, via a first throttle valve, only at higher engine loads. The second exhaust gas turbine can be “slip-started”, via a second throttle valve and while the first throttle valve is closed, by means of excess exhaust gases, instead of these exhaust gases being “wastegated” away. This increases the sensitivity of the second exhaust gas turbine when the first throttle valve opens and the second exhaust gas turbine is fully connected.
A disadvantage of these solutions is that they make poor use of the energy contained in the exhaust gases, since two exhaust manifolds have to be heated up when the engine is started up, which means that it takes longer for a catalytic converter to get up to working temperature.
Another disadvantage is that the load on the different turbines is uneven, which can result in uneven wear to the turbines.